package utils;

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.LongBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;

/**
 * Hash code is based on Media Player Classic. In natural language it calculates: size + 64bit
 * checksum of the first and last 64k (even if they overlap because the file is smaller than
 * 128k).
 */
public class OpenSubtitlesHasher {
        
        /**
         * Size of the chunks that will be hashed in bytes (64 KB)
         */
        private static final int HASH_CHUNK_SIZE = 64 * 1024;
        
        
        public static String computeHash(File file) throws IOException {
                long size = file.length();
                long chunkSizeForFile = Math.min(HASH_CHUNK_SIZE, size);
                
                FileChannel fileChannel = new FileInputStream(file).getChannel();
                
                try {
                        long head = computeHashForChunk(fileChannel.map(MapMode.READ_ONLY, 0, chunkSizeForFile));
                        long tail = computeHashForChunk(fileChannel.map(MapMode.READ_ONLY, Math.max(size - HASH_CHUNK_SIZE, 0), chunkSizeForFile));
                        
                        return String.format("%016x", size + head + tail);
                } finally {
                        fileChannel.close();
                }
        }
        

        public static String computeHash(InputStream stream, long length) throws IOException {
                
                int chunkSizeForFile = (int) Math.min(HASH_CHUNK_SIZE, length);
                
                // buffer that will contain the head and the tail chunk, chunks will overlap if length is smaller than two chunks
                byte[] chunkBytes = new byte[(int) Math.min(2 * HASH_CHUNK_SIZE, length)];
                
                DataInputStream in = new DataInputStream(stream);
                
                // first chunk
                in.readFully(chunkBytes, 0, chunkSizeForFile);
                
                long position = chunkSizeForFile;
                long tailChunkPosition = length - chunkSizeForFile;
                
                // seek to position of the tail chunk, or not at all if length is smaller than two chunks
                while (position < tailChunkPosition && (position += in.skip(tailChunkPosition - position)) >= 0);
                
                // second chunk, or the rest of the data if length is smaller than two chunks
                in.readFully(chunkBytes, chunkSizeForFile, chunkBytes.length - chunkSizeForFile);
                
                long head = computeHashForChunk(ByteBuffer.wrap(chunkBytes, 0, chunkSizeForFile));
                long tail = computeHashForChunk(ByteBuffer.wrap(chunkBytes, chunkBytes.length - chunkSizeForFile, chunkSizeForFile));
                
                return String.format("%016x", length + head + tail);
        }
        

        private static long computeHashForChunk(ByteBuffer buffer) {
                
                LongBuffer longBuffer = buffer.order(ByteOrder.LITTLE_ENDIAN).asLongBuffer();
                long hash = 0;
                
                while (longBuffer.hasRemaining()) {
                        hash += longBuffer.get();
                }
                
                return hash;
        }
        
}
